1. Field of the Invention
The invention relates to electrical filters for use in a system for measuring the time of occurrence of electrical pulses. The filter is amplitude- and rise-time-compensated.
2. Background Art
Electrical circuits that measure the time of occurrence (TOOC) of electrical pulses are known in the art. These electrical pulses are typically unipolar pulses (although bipolar pulses may also exist) characterized by a random amplitude, i.e., an amplitude that may change from pulse to pulse, and a random, non-zero rise time that may also change from pulse to pulse. The pulses are further characterized by being substantially linear over a portion of their leading edge. Generally, a TOOC measurement system comprises a filter network, which shapes the input pulses into bipolar output pulses. Such bipolar output pulses are provided to a level-crossing discriminator, which detects the zero-crossing points of the bipolar output pulses.
In order to accurately measure the time of occurrence of these electrical pulses, it is desirable that the electrical filters produce a bipolar output pulse having a zero-crossing time that is invariant with respect to the TOOC, and independent of the input pulse rise time and amplitude. That is, regardless of the rise time and amplitude of the input pulse, the bipolar output pulse has the same zero-crossing time with respect to the time of occurrence of the input pulse. As will be described further hereinbelow, this invariance ensures an accurate measurement of the time of occurrence of the input pulse.
One previously known amplitude-compensated, rise time compensated TOOC filter is the delay-line constant-fraction filter. This filter shapes the unipolar input pulses into bipolar output pulses wherein the zero-crossing time of the output pulses is insensitive to the amplitude and rise-time changes in the input pulse. The earliest reference to this filter is in the publication by Robert L. Chase, "Pulse Timing System For Use With Gamma Rays On Ge(Li) Detectors", The Review of Scientific Instruments, Vol. 39, No. 9, September 1968, pages 1318-1326. Such filter generally operates as follows. A fraction of the input pulse is provided to one input of a comparator, or differential amplifier. Subtracted from this fraction of the input pulse is a delayed input pulse, delayed by means of a suitable delay line. The output signal thus represents the fraction of the input pulse minus the actual input pulse that is delayed a suitable time period. It can be seen that, regardless of the rise time or amplitude of the input pulse (provided the network parameters are adequately adjusted to ensure that the zero-crossing time occurs during the linear leading edge of the input pulse), the zero-crossing time of the output pulse will remain constant.
The troublesome component in the delay-line constant-fraction filter is the delay line. It is typically fifty feet (50') long, occupies several cubic inches of space, and is time-consuming and expensive to fabricate. Connections to other circuit components require manipulation of a relatively long cable. Because delay lines must be changed physically several times in the course of adjusting the filter to a specific application, a stockpile of delay lines of different lengths is required to adjust the filter to specific applications.
Another filter circuit that provides a bipolar output signal is described in a publication by Kinbara and Kumahara, "A Leading-Edge Time Pickoff Circuit", Nuclear Instruments and Methods 67, (1969), pages 261-266. There, an input-pulse signal is separated into two channels which consist of a delay line and a differentiating RC network. The signals from both the delay line and the differentiating circuit are applied to input terminals of an amplitude comparator so as to determine the time when they are equal. Such circuit maintains the disadvantage of a delay line, as described above.